Andrew Knoll (AK): We’ve known for several years, from orbital data, that there are layered rocks on Mars, but Opportunity gives us our first chance to actually go and work directly on some of these rocks in an outcrop. For geologists, you just can’t overemphasize the importance of that.

The fact that they’re sort of tabular suggests that they’re either fairly thin volcanic deposits or sediments. And the prospect of having in situ sedimentary rocks on Mars that we can go up and interrogate is about a best-case scenario, as far as I’m concerned.

AB: What if they turn out to be volcanic ash deposits? Will that make for a less interesting scenario?

AK: Not at all. I think one of the big questions is, What are the predominant processes that have given rise to layered rocks on Mars? There’s no reason to believe that every layered rock on Mars formed in the same way as the ones that Opportunity’s sitting in front of. But to know even how one of those layered rocks formed will be a step in the right direction.

We will also soon know whether the hematite signal in Meridiani that was detected from orbit is resident in those rocks. Remember the reason that we’re at Meridiani Planum is because of this strong signal for a particular form of iron oxide called hematite. It’s very difficult to think about making hematite without some liquid water interactions with rocks. So even if it’s a volcanic rock, it will help to constrain our thinking about one of the most interesting chemical anomalies on the planet.

AB: There’s a river in Spain, the Rio Tinto, where you’ve spent some time doing research. You’ve suggested that the way the iron minerals at Rio Tinto have degraded and transformed over time might shed some light on how the hematite at Meridiani formed. Can you explain the connection?

AK: Let me start at the beginning. The kinds of thinking we bring to the interpretation of iron on Mars will be informed by our experience with oxidized iron on the Earth’s surface. There are a number of ways that iron deposits have formed on our planet. It may be that no one of them is going to be an exact analog for what happened on Mars. But each of them might give tidbits of information that will help us think about Mars.

Now, Rio Tinto is a very interesting place. It’s in southwestern Spain, about an hour west of Seville, maybe another hour east of the Portuguese border. Rio Tinto is actually of historical interest to people in America since Columbus set sail in 1492 from a port at the mouth of the Rio Tinto. But it’s also of interest to mining geologists because it has been a mine at least since the time of the Romans.

What’s being mined there is iron ore. About 400 million years ago hydrothermal processes formed these iron ore deposits. Mostly the iron is in the form of iron sulfide, or fool’s gold. It’s very rich ore. As rainwater percolates down through these deposits, it oxidizes the pyrite and two things happen. One, it forms sulfuric acid. So the water in the river has a pH of about 1; it’s very acidic. And, two, the iron gets oxidized. So the water is about the color of rubies, because of this iron being carried around.

What’s interesting is that if you look at the deposits that are forming from the Rio Tinto today, most of the iron is coming out as iron sulfate minerals, that is, a combination of iron, sulfur and oxygen; and a little bit of it is coming out as a mineral called goethite, which is iron mixed with oxygen and a little bit of hydrogen. Goethite is basically rust.

That’s not what you see at Meridiani on Mars. But what’s interesting about the Rio Tinto deposit is that this process has been taking place for at least 2 million years. And there is a series of terraces that give us a sense of what happens to these deposits over time.

What we find is that after just a few thousand years, all of the sulfate minerals have disappeared and all of the iron is in this material called goethite. But as you go into older and older terraces, by the time you get to terraces that are 2 million years old, much of that goethite has been replaced by hematite, the mineral on Mars. And it’s a fairly coarse-grained hematite, which is also what we see at Mars.

So the first thing we learn at Rio Tinto is that one doesn’t need to think only about processes that deposit coarse-grained hematite from the get-go. It can form during what geologists call diagenesis. That is, it can form by processes that affect the rocks through time, and it can actually do that at low temperatures and without being deeply buried and subjected to high pressure. So in that sense, Rio Tinto shows us another way in which the hematite in Meridiani could have gotten there. It expands the options we consider.

AB: When geologists say things like "low temperature," they often mean something different than the rest of us do.

AK: When I say "low temperature," I’m talking about the temperatures that you and I experience on a daily basis, room temperature. I would guess that most of the Rio Tinto groundwaters are between 20 and 30 degrees Celsius, maybe 70 to 80 degrees Farenheit.

AB: Does the texture of the rock change over time as a mineral goes through the process of diagenesis?

AK: Yes, it does. Although what’s interesting is that while texture at the level of what the microscopic imager can see definitely changes through diagenetic history, larger scale features of deposition that you would see by looking closely at the outcrop with Pancam appear to be persistent. So, even though the rock is going through these changes, it retains sedimentary signatures of its formation, which is exciting. That’s important.

Color pancam of bedrock outcrop, thought perhaps to be partof the crater rim, about 10 meters from the base stationImage Credit: NASA/JPL

AB: You say that at Rio Tinto you can see a 2-million-year slice that shows you the diagenetic process over time. But the outcrops that Opportunity has seen at Meridiani could be 2 billion years old. Would they still retain any useful information after that long?

AK: Here’s the good news about geology: For sedimentary rocks, in particular, most of the changes that a rock undergoes it undergoes very early in its history. Unless a rock undergoes metamorphism, getting buried and subjected to high pressures and temperature, within at most a few million years of its formation it stabilizes into a form that it will retain indefinitely.

I work, in my day job, on Precambrian rocks on this planet. And I can guarantee you that when I look at a sedimentary rock that’s a billion years old, most of the changes that that rock underwent happened within the first 200 thousand years of its life. And then it stabilizes, and just waits for a geologist.

AB: And we have no reason to believe that physics behaves differently on Mars?

AK: That’s what we have going for us. I’ve said this before in terms of astrobiology: When you’re looking for life beyond our planet, you have no assurance that biology somewhere else will be the same as it is here. But you have pretty good assurance that physics and chemistry will be the same.

AB: Part of what makes Meridiani interesting is that it’s unlike just about any place else on Mars. Even if you’re able to figure out the history of Meridiani, to what extent will you be able to generalize that knowledge to Mars as a whole?

AK: I think it will certainly constrain the way we think about Mars as a whole planet. It may be that, in terms of the overall chemical and rock signature of Mars, that Gusev will turn out to be a better standard-issue Mars surface. That is, most of Mars – in fact, almost all of Mars – is surfaced with basalt, and then covered with fine dust. And that’s what we see at Gusev.

Evidence for recent underground water seepage?Credit: NASA/MOC/THEMIS

Now, it turns out that if you strip away the signal of hematite from the signatures of surface materials in Meridiani that we’ve gotten from orbit, it’s also mainly basalt. So it’s not a completely anomalous part of the planet. It appears to be a representative part of the planet at heart, with this unique hematite signal layered onto it.

One of the features of the Meridiani iron deposit is that, while it’s local with respect to the whole planet, it’s geographically widespread in that you have thousands of square kilometers that give this signature.

Many people think that hydrothermal processes and groundwater processes will give only small local iron signals, but in fact, the hematite-rich layers in the Rio Tinto deposit, go for several thousand square kilometers. Because these groundwaters spread out in a layer over a wide area.

So the Rio Tinto iron deposits do several things that we should keep in mind at Meridiani. They combine ancient hydrothermal and younger low-temperature processes; they need water; they can be layer forming; and they can be widespread.

They’re not the only set of processes that could to that, by any means. I’m not particularly prejudiced in favor of Rio Tinto as a better analog to Meridiani than anything else. I just think that as we go into this exploration we need to at least keep in our memory file as many different products and processes dealing with iron as we can.

All of the different settings for iron deposition and processes of iron deposition we see on this planet carry chemical and textural signals that Opportunity could detect on Meridiani. We can use those comparisons to help us to figure out how the Meridianihematite formed.